1. Field of the Invention
The present invention relates to an ignition system for a multi-cylinder Otto-type four-stroke engine equipped with at least one spark plug for each cylinder, the ignition system including for each spark plug an ignition device connectible substantially without cables to said spark plug in the engine, the ignition system also including a control unit which sends signals in response to the engine operating conditions for triggering sparks at the spark plugs in a given order.
The invention is preferably applicable in a capacitive ignition system including an electric low-voltage block and an electric high-voltage block.
2. Description of Related Art
Capacitive ignition systems have so far not come to any great use on multi-cylinder four-stroke engines propelling automobiles. The reason for this would appear to be that for such engines the dominating inductive ignition systems have been cheaper and have also been able to meet the majority of technical requirements so far placed on them. Known applications of capacitive ignition systems on multi-cylinder Otto-type four-stroke engines have therefore been limited to automobiles intended for sporty driving, i.e. driving to a large extent with high engine load and high engine r.p.m. Under such hard driving, the heat stresses in the combustion chambers of the engine increase, and to avoid so-called knocking during the combustion sequences it is important to use spark plugs with good heat conductivity, so-called cold spark plugs. However, at low engine load the risk of soot deposits and moisture on the insulators of the cold spark plugs increases, and accordingly there also increases the risk that the spark formation between the electrodes of the spark plugs will fail due to creeping currents across the insulators. In comparison with a conventional inductive ignition system, the capacitive ignition system has spark formation with a very short rise time in the ignition voltage, and this not only enables accurate ignition at high load and high engine r.p.m. but also satisfactory ignition for sooty plugs and at low engine load and r.p.m. occurring during idling.
Capacitive ignition systems used up to now on automobile engines have been of a type where the ignition voltage from the ignition coil, also known as ignition transformer, has been taken via a distributor and high-voltage ignition cables to the respective spark plug. Such a design has certain deficiencies, however, since the short rise time to a comparatively high ignition voltage level of the capacitive ignition system causes problems in obtaining satisfactory screening of the high-voltage block of the system to a reasonable cost. For the same reason, the risk of flashover in the distributor and ignition cables also increases and constitutes a limiting factor for the voltage level of the ignition current. Thus, the prospects of producing at high compression pressures a spark which is capable of providing complete ignition of the fuel-air mixture compressed in the cylinder are reduced. The increased risk of flashover furthermore places great demands on carefulness in manual operations on the ignition system, e.g. during service or the like.
In other types of internal combustion engines, e.g. Otto-type two-stroke engines for power saws or the like, it is known to use capacitive ignition systems with an ignition coil connected to the spark plug and built into a unit directly attachable to the spark plug. A capacitive ignition system of this kind lacks ignition timing regulation and furthermore has deficiencies with regard to radio screening, servicability etc.
The present invention has the object of implementing an ignition system of the kind disclosed in the introduction for a multi-cylinder Otto-type four-stroke engine equipped with at least one spark plug for each cylinder, said ignition system including for each spark plug an ignition device connectible to said spark plug in the engine substantially without cables, the invention mainly being distinguished in that the ignition devices for at least two adjacent spark plugs are included in a common, handleable unit, a so-called ignition cassette.
The invention is further distinguished in that the ignition system is of the capacitive type including an electric low-voltage block and an electric high-voltage block, characterized in that the high-voltage block is included in one or more ignition cassettes. Preferably, the ignition devices to all the spark plugs in the engine are included in a single common unit, which enables safe and expedient handling of the unit, while the absence of high-voltage cables and distributors enables good radio screening.
In the inventive solution there is provided a substantially direct connection of the high-voltage block to the spark plugs, which ensures that high ignition voltage with a short rise time can be transmitted to the spark plugs. This enables the spark plugs to be made with large spark gaps without the risk of the sparks failing to jump. Such a relatively large spark gap facilitates complete combustion of the fuel-air mixture in the cylinder, while in particular the content of hydrocarbons (HC) in the exhaust gases may thus be restricted. In an advantageous embodiment of the invention, the engine is provided with spark plugs having a spark gap exceeding 1.0 mm.